Stomach
The stomach is a hollow muscular organ of the GI tract heavily involved in the second stage of digestion following mastication. It is located between the oesophagus and small intestine, and secretes protein-digesting enzymes (proteases), and strong acids to aid in digestion. Following partial digestion here, a liquid called chyme is made, which smooth muscle contortions send through the duodenum and onto the rest of the GI tract Anatomy 'Gross anatomy' The stomach is situated on the upper left of the abdominal cavity. The diaphragm borders it superiorly, the pancreas posteriorly. The greater omentum hangs from the greater curvature. The oesophagus and duodenum bookend the stomach. Folds called rugae cover the internal surface of the stomach. They flatten out to allow the walls of the stomach to stretch in order to accommodate large amounts of food. There are several layers making up the walls of the stomach. The inner lining is the mucosa, comprised of the lamina propria and gastric glands (pits). Beneath the mucosa is the submucosa, muscularis mucosae, and serosa, which is covered by the peritoneum. The muscle layers are not evenly distributed; the external circular muscle layer is relatively thin in the fundus and body, but thick in the antrum to aid the mixing of food, and especially in the pylorus, where it acts as a functional sphincter to regulate emptying of the stomach. The lining of the stomach is protected by a layer of columnal epithelial cells, which have well-developed tight junctions to prevent damage to the underlying tissue from the acidic stomach secretions. They also secrete mucus and alkaline substances to further protect against the harsh acidity. This lining is replaced roughly every week. There are also around 3.5million gastric pits through which the deeper gastric glands secrete substances. The pyloric sphincter separates the stomach from the pylorus. It is not a sphincter, but a functional development of the circular muscle layer. A ring of connective tissue divides the duodenum from the pylorus, allowing contractions in the two areas to be independent of each other. However, the myenteric nerve plexuses that innervate the two are continuous. The stomach is surrounded by parasympathetic and sympathetic plexuses which regulate the secretory and motor activity of the stomach (these networks are named anterior, posterior, superior, and inferior gastric, coeliac, and myenteric) At rest, an adult human stomach has a volume of only 45-75ml, but can expand to hold between 1-1.5L of food. Newborn stomachs can retain only around 30ml. Blood supply to the stomach is a little complex. The lesser curvature is supplied by the right gastric artery inferiorly and the left gastric superiorly (also supplies cardiac region). The greater curvature is supplied by the right gastriepiploic arrtery inferiorly and the left superiorly. The short gastric artery (derived from the splenic artery) supplies the fundus and the upper portion of the greater curvature. 'Histology' As in the other parts of the GI tract, the stomach wall is made up of an outer mucosa, a submucosa, a muscularis externa, and a serosa. Mucosa consists of the epithelium and lamina propria (loose connective tissue), with a thin muscularis mucosae layer separating it from the subucosal layer. The submucosa is a fibrous connective tissue layer containing Meissner's plexus, that separates the mucosa from the next layer; the muscularis externa. The muscularis externa consists of 3 muscular layers. *Inner oblique layer - the only layer not present in the other parts of the digestive system. Responsible for churning and physically breaking down food. Particularly prominent in the antrum, and less so in the fundus. *Middle circular layer - the pylorus is surrounded by a thick circular muscular wall at this level, which is normally tonically constricted to make a functional sphincter to control the movement of chyme into the duodenum. *Auerbach's (myenteric) plexus is found between the middle circular and outer longitudinal layers. Is is responsible for innervating both to facilitate peristalsis and mixing *Outer longitudinal layer A thin layer of cells called serosa covers the whole of the stomach and secretes a serous fluid. It consists of a parietal and visceral layer, much like pericardium. Functions 'Digestion' The stomach's primary function is obviously to aid in digestion of a bolus. The entrance of a bolus stimulates the release of proteases (e.g. pepsin) and hydrochloric acid, which kills or inhibits bacteria and provides the acidic pH that two of the proteases require to function. Peristaltic contractions (beginning at the fundus and moving inferiorly) of the various muscle layers churn the food to eventually create chyme. Chyme passes throug the pyloric sphincter, into the duodenum, and onto the small intestine, where nutrient absorption begins. Processing food to chyme can take anything between 40 minutes to several hours depending on the food (generally proteins take much longer than carbohydrates). The enzymes pepsinongen and prorennin are the inactive precursors of the enzymes pepsin and rennin respectively. They are activated by the low pH of the stomach. Rennin digests the milk protein caseinogen (solube) into casein (insoluble), curdling the milk. Pepsin breaks down a vast range of proteins into polypeptides. 'Secretion and motility regulation' The autonomic nervous system and gastric hormones regulate the flow of certain chemicals into the stomach. Some of the main ones include *Gastrin - causes an increase in hydrochloric acid from the parietal cells and pepsinogen from chief cells and increases motility in the stomach. It is secreted by G-cells in the stomach in response to antrum distension (i.e. from food), and digestive products (especially large amounts of undigested proteins). Inhibited by pH<4 and somatostatin *Cholecystokinin - stimulates gall bladder contractions, decreases gastric emptying, increases release of alkaline pancreatic juice (to neutralise chyme). Synthesised and secreted by I-cells in the mucosal epithelium of the small intestine. *Secretin - decreases gastric acid release and motility whilst increasing the release of alkalie pancreatic juices. Secreted by the S cells of the duodenum. *Gastric inhibitory peptide - decreases gastric acid release and motility. Synthesised and secreted by K cells in the duodenum and jejeunum. *Enteroglucagon - decreases gastric acid release and motility. Secreted by mucosal cells of the terminal ileum and colon. Has a precursor called preproglucagon. Other than gastrin, these hormones all act to inhibit the normal action of the stomach. This is to allow the liver and gall bladder time to absorb and further break down unabsorbed food products. Whilst the intestines are busy digesting and absorbing nutrients, the stomach is merely storage.